Second higher harmonic generation utilizing nonlinear optical effects can convert coherent light outputted from a fundamental wave light source into a second higher harmonic having a half wavelength. In the wavelength conversion, when phase matching is non-critical, the conversion efficiency is improved by an increase in element length or by an increase in power of an incident fundamental wave. Accordingly, high output power short-wavelength light generation becomes possible. In wavelength conversion using potassium titanate phosphate (KTiOPO4:KTP) as a nonlinear optical crystal, an increase in power density of the fundamental wave or the higher harmonic causes generation of color center that is called as a gray track, thereby resulting in absorption of the fundamental wave or the higher harmonic. When a gray track is generated, heat distribution occurs due to the above absorption and the nonlinear crystal is subjected to destructive damages. As a way to solve this problem, there have been proposed methods for obtaining the second higher harmonic separated from the divided crystal regions as shown in patent document 1.
FIG. 17 is a diagram illustrating a schematic structure of a conventional short wavelength light source.
In FIG. 17, a part of the fundamental wave 1006 that is outputted from a fundamental wave light source 1001 is wavelength converted by a KTP 1002, and is separated into the fundamental wave 1006 and a higher harmonic 1007 by a wavelength separation mirror 1004. Then, a part of the fundamental wave 1006 is again wavelength converted by the KTP 1003, and is separated into the fundamental wave 1006 and a higher harmonic 1008 by a wavelength separation filter 1005.
In this way, a second higher harmonic is taken out by using a plurality of KTP crystals in a state where the power density of the generated second higher harmonic does not exceed a certain value. Thus a higher efficiency is realized. Patent Document 1: Japanese Published Patent Application No. Hei. 11-271823